Best way to get a list of modifiers in Python?
bpy.types.Modifier.bl_rna.properties['type'].enum_items contains a list of all modifier types. 

To get a list of their Python identifiers you can do:

modifiers = []

for modifier in bpy.types.Modifier.bl_rna.properties['type'].enum_items:

    modifiers.append(modifier.identifier)


Or, more Pythonically:

modifiers = [modifier.identifier for modifier in bpy.types.Modifier.bl_rna.properties['type'].enum_items]